The present invention relates to an improved apparatus for coupling a tool element, such as a grinding wheel, to the output spindle of a power tool, such as a portable grinder. Additionally, the present invention relates to an improved method of manufacturing a tool element, such as a grinding wheel.
The grinding wheel used on portable grinders generally consists of an abrasive disc having a centrally located bore for receiving an internally threaded collar nut. The collar nut has a hub portion that fits into the bore in the grinding wheel so that the enlarged hex-shaped head portion of the collar nut abuts the underside of the grinding wheel. The collar nut is adapted to be mounted to the externally threaded spindle of the grinder. Typically, a support flange is positioned on the spindle between the grinding wheel and an annular shoulder formed on the spindle to provide backing support for the grinding wheel. The support flange is typically comprised of a metal stamping that is configured to engage the backside of the abrasive disc around its outer radial end. The direction of rotation of the spindle when the grinder is energized is such that the collar nut will self-thread onto the spindle until a tight frictional engagement is provided between the support flange and the grinding wheel. The grinding wheel can then be further tightened onto, or subsequently removed from, the spindle by applying a wrench to the collar nut.
With conventional abrasive disc subassemblies the central bore in the abrasive disc through which the spindle extends is circular in shape. Similarly, the hub portion of the collar nut that fits into the bore of the disc is also circular in cross-section. The collar nut in such conventional assemblies is not permanently affixed to the abrasive disc, but rather is intended to be reused when a worn disc is replaced. In addition to the possibility of losing or misplacing the collar nut, this type of assembly is further disadvantageous from the standpoint that replacement abrasive discs must have properly sized bores, which are not uniform for all brands and models. Moreover, the application of driving torque from the spindle to the abrasive disc is solely through the frictional interfaces between the abrasive disc and the spindle directly or between the abrasive disc and the supporting flange and the supporting flange and the spindle. Consequently, under load the abrasive disc subassembly may slip at either of these frictional interfaces. To combat slippage, abrasive disc subassemblies are frequently tightened onto the spindle to such a degree that subsequent removal becomes difficult.
To alleviate these problems, various "hubbed"-type abrasive disc subassemblies have been proposed, such as that shown in U.S. Pat. No. 4,494,615 to MacKay, Jr. Hubbed-type abrasive disc subassemblies include a backing flange that is permanently affixed to the backside of the abrasive disc by the hub portion of the collar nut which thus becomes an integral part of the subassembly. The entire subassembly is thus intended to be discarded when the disc is worn. Hubbed-type grinding wheels are generally intended to be used in combination with specially designed support flanges adapted for engaging driving surfaces on the backing flange affixed to the disc.
With each of the known forms of grinding wheel subassemblies, driving torque is transferred from the output spindle of the grinder to the grinding wheel via a frictional coupling, either between the output spindle and the grinding wheel directly, or through an intermediary support flange which either frictionally engages the backside of the grinding wheel or a backing flange permanently affixed thereto. Frictional couplings of the above-described type without support flanges are prone to slippage, or in the alternative, must be tightened to such a degree as to subsequently make it difficult to remove a worn wheel. While the hubbed-type grinding wheels are much less susceptible to slippage problems, they are substantially more expensive than conventional non-hubbed grinding wheels and consequently are not as widely used.
Accordingly, there is need for an improved grinding wheel subassembly that provides a positive means of coupling the grinding wheel to the spindle of the grinder without the expense of the hubbed-type wheel subassemblies. In addition, it is desirable to provide such an improved grinding wheel subassembly that can be readily manufactured as a hubbed or a non-hubbed grinding wheel and can be used with or without a support flange.
Furthermore, it is desirable to provide a grinding wheel subassembly that is compatible with both United States and European safety standards.